The rapid increase in the use of fossil fuels has accelerated the demand for alternative energy sources or clean energy sources, and research has been actively carried out on power generation and power storage using electrochemistry.
A typical example of an electrochemical device using such electrochemical energy is a secondary battery, which has been increasingly used in various fields.
Based on the shape of a battery case, secondary batteries may be classified into a cylindrical battery having an electrode assembly mounted in a cylindrical metal can, a prismatic battery having an electrode assembly mounted in a prismatic metal can, and a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case made of an aluminum laminate sheet.
The electrode assembly mounted in the battery case functions as a power generating element, having a positive electrode/separator/negative electrode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly-roll type electrode assembly configured to have a structure in which a long sheet type positive electrode and a long sheet type negative electrode, to which active materials are applied, are wound in a state in which a separator is disposed between the positive electrode and the negative electrode, or a stacked type electrode assembly configured to have a structure in which a plurality of positive electrodes having a predetermined size and a plurality of negative electrodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the positive electrodes and the negative electrodes.
As an electrode assembly with an improved structure, in which the jelly-roll type electrode assembly and the stacked type electrode assembly are combined, a stacked/folded type electrode assembly has been developed. The stacked/folded type electrode assembly is configured to have a structure in which a plurality of full cells, each of which has a predetermined size and is configured to have a positive electrode/separator/negative electrode structure, or a plurality of bicells, each of which has a predetermined size and is configured to have a positive electrode (negative electrode)/separator/negative electrode (positive electrode)/separator/positive electrode (negative electrode) structure, are folded using a long continuous separation film.
In addition, in order to improve the processability of a conventional stacked type electrode assembly and to satisfy the demand for various kinds of secondary batteries, there also has been developed a laminated/stacked type electrode assembly configured to have a structure in which unit cells, each of which includes electrodes and separators laminated in a state in which the electrodes and the separators are alternately arranged, are stacked.
Meanwhile, the demand for a lithium secondary battery, which is one of the types of the secondary battery, has increased since the lithium secondary battery is lighter than the other secondary batteries and has higher energy density than other secondary batteries. Even though consumers' demand for a battery having high energy density has steadily increased, the energy density of the lithium secondary battery is still not enough to satisfy the consumers' demand.
In order to solve the above problem, an attempt to increase the energy density using the space inside the secondary battery has been made. However, desired effects have not been achieved due to structural limitations of the secondary battery.
Particularly, in a battery configured to have a structure in which a plurality of electrode plates and a plurality of separators are stacked, many attempts has been made to reduce the volume occupied by electrode tabs protruding from the electrode plates and the volume occupied by a coupling part for connecting the electrode tabs and an electrode lead. However, these desired effects have not been achieved either.
Therefore, there is a high necessity for technology that is capable of improving the energy density of a battery configured to have a structure in which a plurality of electrode plates and a plurality of separators are stacked while maintaining the overall structure of the battery.